Is it worth the effort to root the router and install openwrt especially with the limitation mentioned below? The only reason why I am looking at OpenWRT is so that I can use modern VPN protocols on the router since the default firmware only supports L2TP and PPTP
I would say NO although you can rollback easily from OpenWRT to Stock Firmware.
I had very poor experience with the roaming and I always felt Stock Firmware was way better on this regards. At the end of the day, if you can customize the Stock Firmware config files (again this is OpenWRT under the hood but probably older version), you can get the desired balance.